The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers may incorporate microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers are programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application No. PCT/US85/02010, entitled Stimulated Heart Interval Measurement, Adaptive Pacer and Method of Operation, assigned to the assignee hereof. The disclosure of that application is incorporated herein by reference.
In order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, these contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram and include a P-wave, representing the depolarization of the atria; the QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but the main reason why atrial repolarization is not seen is that the muscular mass of the atrium is so much smaller, compared to the ventrical.)
Thus, it is the P-QRS-T cycle of waves that represents the natrual AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional surface or non-intracardial ECG techniques whenever the operation of the heart is being examined.
Initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall. The SA node depolarizes spontaneously at an intrinsic rate of a little better than once each second (typically about 72 beats per minute). The rate of depolarization and, therefore, the heart rate are influenced by various physical factors, such as nerval (parasympaticus) or hormone stinulation. These factors increase heart contraction rate from levels of 50 beats per minute at rest up to 180 beats per minute under work load.
Optimally, in a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. A short time later (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 120 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 650 ms. Thus, it is an atrial contraction (A), followed a relatively short time thereafter by a ventrical contraction (V), followed a relatively long time thereafter by the next atrial contraction, that produces the desired AV synchrony. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump (largely because the ventricle is contracting when it is not filled with blood).
Multiple-mode, demand-type, cardiac pacemakers are designed, insofar as is possible, to maintain an AV synchrony for damaged or diseased hearts that are unable to do so on their own. A demand-type pacemakers is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval. In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Hence, by first sensing whether a natural depolarization occurs in each chamber, and by second stimulating at controlled time intervals each chamber with an external stimulation pulse in the absence of a natural depolarization, the AV synchrony of the heart can be maintained. Thus, with a demand pacer, the heart will either beat on its own (without stimulation from the pacemaker) at a rate that is at least just slightly faster than the stimulation rate defined by the escape interval, or the heart will be stimulated by the pacer at a rate controlled by the escape interval. The stimulation rate provided by the pacemaker is typically referred to as the "programmed rate".
Unfortunately, there are many operating constraints and conditions of the heart that complicate the operation of a demand-type pacemaker. For example, there are certain time periods following a depolarization of cardiac tissue (prior to repolarization) when the application of an external electrical impulse is ineffective--that is, it serves no useful purpose, and thus represents an unneeded expenditure of the pacemaker's limited energy. Therefore the application of stimulation pulses during these time period is to be avoided.
Rate responsive pacemakers employ some type of physiological sensor for sensing a change in the metabolic needs of a patient. This sensed change, in turn, is used to adjust the rate at which stimulation pulses are delivered to the heart of the patient by the pacemaker. Thus, as the metabolic needs of the patient increase--indicating a need for the heart to beat faster--the rate at which the pacemaker stimulates the heart is increased as a function of this sensed increase in metabolic need. As the metabolic needs of the patient decrease--indicating a need for the heart to beat slower--the rate at which the pacemaker stimulates the heart is correspondingly decreased.
In a demand pacer, the physiological sensor (which may be one of numerous types) adjusts the pacing rate by adjusting the escape interval of the pacer. As the escape interval is thus adjusted as a function of sensed physiological need, the rate at which stimulation pulses are provided to the heart--and hence the heart rate--is correspondingly varied as a function of sensed physiological need.
Rate-responsive demand pacers are typically single chamber pacers that sense and stimulate in the ventricle (VVI) at a rate determined by the particular phsiological sensor used. Patients who are candidates for rate-responsive pacing usually include patients exhibiting partial or complete heart block. When heart block exists, the ventricle does not consistently follow the atrium, and the needed and desired AV synchrony is lost. A rate-responsive pacer advantageously allows the ventricle to be stimulated at a rate commensurate with the sensed physiological need despite the existence of heart blockage.
Fully automatic, dual chamber pacemakers (DDD) are capable of sensing and pacing in both the atrium and ventricle. Such a pacemaker combines the feature of AV sequential pacing with stimulation of the ventricle in response to atrial sensing. Such a pacemaker may also provide stimulation at a pre-programmed minimum rate.
It is customary in such pacemakers to incorporate an input signal sensing detector which may be used to modify or inhibit pacemaker reaction to a sensed physiological signal. For example, in the pacemaker, the electrophysiological signal from a heart contraction is received, amplified and processed in order to suppress noise and other extraneous signals. The resulting signal is fed into the input sensing detector. The output from the sensing detector may be used to inhibit pacemaker stimulation when normal heart activity is detected (VVI) or to synchronize pacemaker stimulation to the heart activity (DDD), depending on pacemaker operation mode.
In these applications, a certain minimal amount of input signal energy (signal level) is required to activate the detector. The sensing detector activating level (sensing threshold) may be fixed--i.e., preset--or adjustable (programmable). In the latter case, the adjustability is usually selected in discrete steps. For whatever threshold level is determined, an output signal from the sensing detector is an indication that the threshold has been exceeded by a processed heart signal. However, the amount of energy or signal extent above the threshold cannot usually be determined in presently known systems.
The extent by which the input signal exceeds the threshold is defined as the "sensing margin". If the input signal exceeds the threshold significantly, the sensing margin is defined as high. If the input signal just reaches or barely exceeds the detector threshold level, the sensing margin is low or zero. For a given level of input signal to the sensing detector, sensing margin is determined by adjusting the detector threshold level.
In order to assure safe operation of an implanted pacemaker, it is important to establish a correct sensitivity setting or threshold level. The proper sensitivity setting of the pacemaker may be affected by a number of factors. For example, the sensing properties of the pacemaker as implanted may be different from those of the measuring equipment preliminarily used to determine sensitivity level. Also, the sensing properties of the pacemaker may be changed after implantation by programming of the device. There may also be a change in the sensed signal property after implantation as a result of physiological changes over time. It is therefore considered important to to be able to determine the optimum threshold level setting of the sensing detector for the actual implanted pacemaker with its actual sensing properties and control settings. An implantable pacemaker having the capability of determining the sensing margin and adjusting sensing detector threshold to an optimum level after implantation will significantly increase the safety and ease of use of the pacing system.
In the field of pacemaker technology, circuitry is known which will respond to electrical signals derived from detected heart activity signals and provide an output signal indicating detection of a signal exceeding a predetermined threshold level. Such circuitry is disclosed in German patent document P 32 32 478.2, corresponding to U.S. Pat. No. 4,516,579, as comprising a differentiator stage, peak detector stage, summing amplifier stage and comparator stage all connected in series. The input to the differentiator stage is coupled to receive signals derived from detected heart activity. One input to the comparator stage is coupled to a threshold voltage level Vref. When the summing amplifier input to the comparator stage exceeds the threshold voltage Vref, corresponding to the positive and negative slopes of a heart-derived signal differing by more than a certain amount, the output of the comparator stage goes high, thus indicating heart signal detection.